Decline in local abundance of bottlenose dolphins (Tursiops truncatus) in the Bay of Islands, New Zealand

Abstract

Regional populations of bottlenose dolphins (Tursiops truncatus) around New Zealand are genetically isolated from each other and the species was recently classified as nationally endangered based on relatively small population sizes and reports of high calf mortality. Here, we estimate the abundance and trends in one of these regional populations, the Bay of Islands, using a photo‐identification database collected from 1997 to 1999 and from 2003 to 2006, containing a total of 3,841 records of 317 individual dolphins. Estimates of abundance obtained with the robust design fluctuated widely but showed a significant decline in the number of dolphins present in the bay over time (7.5% annual rate of decline). Temporary emigration was random and fluctuated considerably (γ = 0.18, SE = 0.07 to γ = 0.84, SE = 0.06). Apparent survival was estimated at 0.928 (CI = 0.911–0.942). Seasonal estimates (26 seasons) obtained in POPAN also showed a significant decline in abundance (5.8% annual rate of decline). Despite the decline observed in local abundance, dolphins continue to be found regularly in the Bay of Islands, suggesting that fewer dolphins use the bay on regular basis. Consequently, it seems that a change in habitat use, mortality and possibly low recruitment could underlie the apparent local decline.

Overlooking a potential hotspot at Great Barrier Island for the nationally endangered bottlenose dolphin of New Zealand

“…Similarly, some dolphins may only visit the area rarely (i.e. transient dolphins), something that has also has been noted for BOI (Tezanos-Pinto et al 2013).…”

“…Mann et al (2000) found bottlenose dolphin group sizes were largest for groups with calves up to 3 mo old, which may also in part explain the formation of large groups at GBI given the high frequency of neonates and calves recorded. Based on the variable patterns of site fidelity and habitat use observed in BOI (Constantine 2002, Tezanos-Pinto 2009, Hartel et al 2014, Tezanos-Pinto et al 2013, we anticipated new individuals would continue to be photo-identified for the duration of the study. The discovery curve reached a plateau after 17 mo of field effort, although a further 8 new individuals were added to the catalogue in the final summer season.…”

“…1) (Constantine 2002); although infrequent sightings of known individuals outside of these areas have more recently extended the known range to the west coast of the North Island (Tezanos-Pinto et al 2013). A decline in abundance has been reported for the Bay of Islands (BOI) (Tezanos-Pinto et al 2013), where studies on bottlenose dolphins have formed the basis of our understanding of the North Island population.…”

“…1) (Constantine 2002); although infrequent sightings of known individuals outside of these areas have more recently extended the known range to the west coast of the North Island (Tezanos-Pinto et al 2013). A decline in abundance has been reported for the Bay of Islands (BOI) (Tezanos-Pinto et al 2013), where studies on bottlenose dolphins have formed the basis of our understanding of the North Island population. Research on the species in this region has primarily been driven by funding generated from dolphin tourism levies through the Department of Conservation (DOC), the government agency responsible for managing New Zealand's marine mammal species (e.g.…”

“…Under the New Zealand Threat Classification System, the species was listed as 'Not Threatened' up until 2002 (Hitchmough 2002) and subsequently uplisted to 'Range Restricted' in 2005 (Hitchmough et al 2007). A further reclassification to 'Nationally Endangered' in 2009(Baker et al 2010) was based on apparent declines in abundance in 2 of the 3 coastal populations, coupled with reports of high calf mortality (Currey et al 2009, Tezanos-Pinto et al 2013.…”